Abstract
The surface-recorded brain stem electric responses to unfiltered clicks were studied in 60 patients with sen-sorineural hearing loss: 48 patients with bilateral hearing loss which was practically symmetrical on the two sides and 12 patients with asymmetrical hearing loss with pronounced side difference. Among the patients with symmetrical hearing loss, 1/4 had an audiogram of flat loss type, 1/4 had gradually sloping audiograms, 1/4 had steeply sloping audiograms and the last 1/4 consisted of a mixed group with diverse types of audiogram. The patients with asymmetric hearing loss represented various inter-aural combinations of hearing threshold curves. All patients were examined using clicks presented monaurally and clicks presented binaurally, simultaneously, at several stimulation levels. The amplitude was measured as the peak-to-peak value of the Jewett5-FFP7 complex, and the latency was measured to the peak of FFP7. Among the results obtained might be mentioned that binaural stimulation was found to be superior to monaural stimulation, as the bilateral stimulation resulted in a significant (approx. 40%) increase in amplitude and often permitted the response to be recorded down to a slightly lower level. Even in patients with asymmetrical hearing did bilateral stimulation yield clear-cut responses. Further, that it was usually possible in patients with flat loss to trace the responses farther down towards the subjective threshold than was the case in patients with gradually sloping or steeply sloping audiograms, and that the responses in the first group were usually more clearly defined. In the flat loss patients (who all had relatively good hearing at the higher frequencies), the latency was found to be shorter than in the patients with pronounced high frequency loss (the groups with gradually sloping and steeply sloping audiograms). In patients with asymmetrical hearing loss the response amplitude exhibited a recruitment-like phenomenon, as the amplitude as a function of stimulus level increased most rapidly in stimulation of the inferior ear. It may be concluded that as was to be expected, the brain stem responses to unfiltered click stimuli in patients with cochlear dysfunction reflect in the first instance the function of the basal parts of the cochlea. The uncertainty in the use of unfiltered clicks as the sole stimuli in brain stem audiometry is so great that the procedure must be regarded as inadequate. The investigation should be supplemented by the use of other stimuli.